Python – Group Sublists by another List

Sometimes, while working with lists, we can have a problem in which we need to group all the sublists, separated by elements present in different list. This type of custom grouping is uncommon utility but having solution to these can always be handy. Lets discuss certain way in which this task can be performed. Method #1 : Using loop + generator(yield) This is brute force way in which this task can be performed. In this, we iterate the list and make groups dynamically using yield. We keep track of elements occurred and restart list when we find element in second list. 

Python3

# Python3 code to demonstrate
# Group Sublists by another List
# using loop + generator(yield)

# helper function
def grp_ele(test_list1, test_list2):
    temp = []
    for i in test_list1:
        if i in test_list2:
            if temp:
                yield temp
                temp = []
            yield i
        else:
            temp.append(i)
    if temp:
        yield temp

# Initializing lists
test_list1 = [8, 5, 9, 11, 3, 7]
test_list2 = [9, 11]

# printing original lists
print("The original list 1 is : " + str(test_list1))
print("The original list 2 is : " + str(test_list2))

# Group Sublists by another List
# using loop + generator(yield)
res = list(grp_ele(test_list1, test_list2))

# printing result
print ("The Grouped list is : " + str(res))

Output : 

The original list 1 is : [8, 5, 9, 11, 3, 7]
The original list 2 is : [9, 11]
The Grouped list is : [[8, 5], 9, 11, [3, 7]]

Time Complexity: O(n*n)
Auxiliary Space: O(n)